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CONSTRUCTION KNOWLEDGE  >> MASONRY >>

MASONRY
 

1.  What is Masonry?
2.  What Types and Sizes of Concrete Masonry Units are Available?
3. What Types and Sizes of Bricks are Available?
4. How Do Lintels Work?
5.  How Does Steel Reinforcing Get Used in Masonry?
6.  What Should I Know about Mortar and Grout?
7.  Why are Movement Control Joints So Important?
8.  What Public Domain Documents are Available for Further Study?
9. Tricks of the Trade & Rules of Thumb for Masonry:

What is Masonry?

Masonry consists of building structures by laying individual masonry units (brick, concrete block, stone, etc). Normally the masonry units are laid with cement mortar, which binds them together to create a structure. Masonry construction can provide beautiful walls and floors at economical prices. Due to the individual masonry units, masonry construction tends to be quite labor intensive. Because of the cement, clay or stone nature of the masonry materials, masonry construction tends to be durable and often requires little maintenance.  An excellent introduction to the basics skills and concepts of masonry work is found in the US Navy Builder Training Course Volume #1.

Similar to concrete, masonry tends to be high in compressive strength but low in tension strength. To get a better understanding of the structural advantages and challenges of masonry, see the section on Structural Basics for Concrete. Crack control tends to be a major concern in masonry structures and normally is addressed by Design Professionals on the Plans.

What Types and Sizes of Concrete Masonry Units are Available?

Concrete masonry units (CMU) are some of the most commonly used materials in building construction. Due to their relatively low cost as wall material and their fire resistance, CMU walls are laid on many types of projects. From schools to warehouses, many buildings have CMU walls. It's valuable to understand the types and sizes of CMUs commonly available. Since CMUs are produced by private manufacturing firms, there is really no limit to the various of CMU types and sizes that could be produced. The following link shows commonly available CMU types and sizes from one firm: http://www.newhollandconcrete.com/images/images9021/Products/Gray/graysizes.pdf.

You may want to consider printing out these shapes and sizes if you want to better understand what options the Masonry Contractor has for completing their portion of the project. Or, if you have convenient web access, you can just come back to this link as needed.Many specialty CMU products are also available:

• Split face
• Ground face
• Sand blasted
• Matte face
• Scored and ribbed
• Light weight
• Acoustical
• Insulated

Specifics about types and sizes of these various CMUs can be found at the following website:  http://www.newhollandconcrete.com/default.aspx?portalid=9021&tabid=172&pageid=Arch_Msnry

What Types and Sizes of Bricks are Available?

Bricks are a hardened clay product used for over 5,000 years. The Bible story about Moses and the Egyptian Pharaoh discusses the making of bricks by the Israelite slaves. The ancient brick size was a length to width to depth ratio of 4:2:1, which is still common today.

Modern brick are typically manufactured in one of the following processes: soft mud, dry press or wire cut. In the soft mud process, the clay and sand are mixed with water, and often lime or ash, to a soft, mud-like consistency then pressed into steel molds by hydraulic presses. The bricks then are fired in ovens to increase strength. This process tends to be the lowest cost..

The dry press process seems similar to the soft mud process, except the process uses a thicker, drier clay mix and more hydraulic power to push the mix into the molds. The dry press process achieves more defined edges and consequently costs a bit more.Brick color comes from the heat of the firing and changes with various added minerals. Sample brick panels are often laid on construction projects to show the variation of brick colors (and mortar colors and type of joint), so the final product on the building matches the approved sample.

The following website shows lots of brick types and sizes:  http://www.glengerybrick.com/brick/index.html

How Do Lintels Work?

As mentioned above, masonry and concrete tend to be strong in compression but weak in tension. In olden times, builders took advantage of this fact by the use of arches above door and window openings. Arches are an interesting structural form because they act totally in compression. Therefore, arches above windows have the downward load of the wall and floor/roof loads above transferred around the arch keeping the structure in compression, so tension cracks didn’t occur in the masonry arch. Barrel vault ceilings are really just three dimensional arches, so they also worked as compression members only..A lintel is defined as a horizontal structural member supporting the wall load above a window or door. Lintels can be precast concrete, steel structural shapes, stone, wood or other products. Some common lintel types are shown below. The Construction Supervisor should be aware of the concept of arch action with regard to lintel design. A lintel designed for arch action only needs to carry the actual wall weight in the 45 degree angle triangle directly above the opening. Since typical precast CMU lintels are designed this way, if a change that places a concentrated load in this triangle happens during the course of the project, the Construction Supervisor should understand the ramifications and point it out to the Design Professional.

Figure 4.1

Construction Supervisors should also pay attention to the likelihood of lintels rusting and discoloring the facade. On many projects, steel lintels rust through the paint after a year or two and look bad. Sometimes steel lintels are required to be galvanized or factory powder coated, in these cases the lintels only need to be protected from scratches before and after installation. If no coating is specified, it's not uncommon for primed steel, or even rusty steel to be installed and painted after it's in place. Since a lintel can't be effectively painted after it's installed, the Construction Supervisor should attempt to make sure lintels are properly primed and painted prior to installation.

How Does Steel Reinforcing Get Used in Masonry?

Since CMUs and mortar have high compression strength but low tension strength, manufacturers have determined many steel products to help achieve stronger and more serviceable walls. Rebar can be added in masonry walls, with cores grouted solid, to substantially increase the load carrying capacity of a wall. Generally the Structural Engineer designates the size of the rebar and the spacing. Since CMU cores are usually on 8" centers, the common spacing is 8", 16", 24", 32" or 48" on center.

There are also specialty CMU products that allow rebar to be run horizontally. Bond beam are designed for this purpose, as are open ended CMUs. When using these products, grout bonds the rebar to the CMUs making an integral wall. An important item for the Construction Supervisor to watch concerns the grout used in this reinforced masonry. Many times the Structural Engineer specifies the grout to be a concrete grout, say 3,000 psi, with pea size aggregate. It's easiest for the Masonry Contractor just to use the same mortar he's using to lay the wall. If no one pays attention, a 1,200 psi mortar may be installed instead of the specified 3,000 psi concrete grout. Obviously, this becomes a difficult problem to go back and fix.

In order to get an understanding of the various wall ties, joint reinforcement and other masonry products available, the following website provides excellent information: http://www.dur-o-wal.com/prod/index.html. Generally the Project Documents should show the required information, but it's good to know what is available.

What Should I Know about Mortar and Grout?

Mortar binds the masonry units together. Commonly made from a mixture of Portland Cement and lime, sand and water, mortar mixes are usually specified for in the Contract Documents for a project. There are mortar systems that are produced offsite and delivered to the jobsite (Ready-Mixed Mortars), but most mortar gets mixed in the field. Due to the field mixing of mortar, it's important the mortar ingredients and mix be consistent throughout the project. While this responsibility typically falls to the Masonry Contractor, the Construction Supervisor should notice if the mixing process seems to proceed in a quality fashion. If ingredients are stored such that foreign elements become mixed into the mortar, if the measuring process seems haphazard from one batch to the next, or if the consistency of the mortar seems to vary between batches, the Construction Supervisor may head off future problems by noticing these issues and discussing with the Masonry Contractor. The Construction Supervisor can't catch every problem on a job site, but can pay attention to best quality practices and develop the habit of noticing where future problems are likely to arise.

Mortars types are typically Type M, S, N or O. A basic understanding of these mortar types is helpful. All mix quantities given below are volume proportions (as opposed to weight proportions).

Type M mortar is the highest strength (2500 psi average compressive strength at 28 days) and is often used for below grade structures such as foundations walls and storm water structures. It has typical proportions being 1 part Portland Cement, 1/4 part lime and 6 parts sand.

Type S mortar is the next highest in strength (1,800 psi average compressive strength at 28 days) and is the general purpose high strength mortar. It tends to be the highest cost of the mortar mixes and has proportions of 1 part Portland Cement, 1/2 part lime and 4 1/2 parts sand. Most reinforced masonry walls will use Type S mortar and it's also specified for many other uses.

Type N mortar is a medium strength mortar (750 psi average compressive strength at 28 days) and tends to be used for above grade walls in moderate wind areas. The workability of Type N mortar is quite high due to the higher proportion of lime in the mix. Typical mix quantities are 1 part Portland Cement, 1 part lime and 6 parts sand. Type N and Type M mortars are lower cost than Type S mortar.

Type O mortar is a low strength mortar (350 psi average compressive strength at 28 days) used only for non-load bearing interior walls. It is the lowest cost mortar with mix proportions of 1 part Portland Cement, 2 parts lime and 9 parts sand. It is rare to see Type O mortar specified, but it is allowable to use per the International Building Code 2006. A more detailed understanding of mortar properties can be obtained with the following free download from the Portland Cement Association: http://www.cement.org/bookstore/profile.asp?store=&id=262

Grouts are either fine grouts (Portland Cement, lime and sand) or coarse grout (Portland Cement, lime, sand and coarse aggregate). Typically the Structural Drawings define the type of grout required and the properties: strength, maximum aggregate size, etc. Since Masonry Contractors often seem to use the currently mixed mortar for grout, because it's easier, the Construction Supervisor should make sure a clear understanding exists as to the grout requirements.

The grouting method can be low lift grouting or high lift grouting. Low lift grouting is a simple method of placing grout at scaffold height (prior to building the next lift of scaffold) or bond beam height. Vertical rebar, if required, are often placed in the cores after grouting and stirred to help consolidate the grout. The bar lap for the vertical rebar is often a minimum of 30 bar diameters. One difficulty with low lift grouting involves the lifting of the next courses of concrete masonry units over the rebar dowels that project after each grout lift.

High lift grouting, on the other hand, allows the Masons to grout the wall for the entire story (up to 24') and is more complex. The size of the open vertical cells needs to be evaluated and 3" x 4" clean out openings at the bottom of the grout lift should be used. The high lift grouting method makes economical use of the grout pump and minimizes laps in vertical rebar, which both add to overall jobsite efficiency.

With either grouting method, making sure grout actually fills the wall voids designated to be filled is an area often missed on job sites. There should be a method agreed upon for filling wall voids that assures the grout gets all the way to the bottom. The Construction Supervisor should be aware of the likelihood of failure in this area and take steps to make sure the process is done correctly.

Why are Movement Control Joints so Important?

Buildings move. Sometimes movements come from temperature related expansion and contraction. Or perhaps structural loads from wind, snow, or vibration get movement started. Of course, foundation settlement also moves buildings. Due to the tendency for masonry to crack easily (low tension strength), movement control must be considered for proper construction of masonry walls.

One common method of dealing with masonry wall cracking is to improve the tensile strength of the wall. Bond beams utilize special cmu shapes that allow rebar to be placed horizontally and grouted full. Steel horizontal joint reinforcement also can be used at 8" to 24" on center and laid directly in the horizontal masonry joint. Different styles of horizontal joint reinforcement can be found at: http://www.dur-o-wal.com/prod/index.html.

Control joints are used to relieve masonry tensile stresses and allow movement to occur. Typically a pre-formed rubber gasket is used at vertical control joints to transfer the shear load (normally wind load) across the joint but still allowing the joint to move horizontally. Masonry control joints should be shown in Contract Documents. The following table gives guidelines for control joint spacing:

Control Joint Spacing for Concrete Block Construction
(Type 1, moisture controlled units)
Horizontal Joint Reinforcement, vertical spacing
 

What Public Domain Documents are Available for Further Study?

This US Department of Army Concrete and Masonry  Field Manual does a great job explaining Concrete and Masonry basics. It's 323 pages of figures, basic explanations and instructions of how to do the work. If you are somewhat new to construction, take some time and review this excellent resource. The official name is US Army FM 5-428.

Another great all around great construction introductory guide that covers some masonry work is the US Navy Builder Training Course Volume #1. The official name of this 332 page resource is Builder 3 and 2, Volume #1, NAVEDTRA 14043.

The US Dept of Defense Masonry Structural Design for Buildings is a mostly engineering design handbook, but has some great simple design guides in the appendix. The official name of this 169 page handbook is UFC 3-310-05A, 1 March 2005.

The US Dept of Defense Cold-Formed Load Bearing Steel Systems and Masonry Veneer/Steel Stud Walls is a design handbook that has information regarding masonry veneer walls. The official name of this 148 page handbook is UFC 3-310-07A, 19 June 2006.

Tricks of the Trade & Rules of Thumb for Masonry:

1. Understand that lintels are normally designed to only carry the triangle of wall above them and notice if loads are applied that exceed lintel capacity.
2. Make sure steel lintels, if not galvanized or factory powder coated, are properly primed and painted on all surfaces prior to installation (since lintels can't be effectively painted after installation).
3. Think about temporary bracing of masonry walls as a structural engineer would.
4. CMU walls may span 18 times their thickness (i.e. 18' for an 12" thick wall) for load bearing and exterior non-load bearing walls or 36 times their thickness for interior non-load bearing walls. Empirical masonry design IBC 2003, 2109.4.
5. Portland Cement is normally sold in 94 pound bags because one cubic foot of Portland Cement weighs 94 pounds.
6. Make sure mortar doesn't get used when grout is specified.
7. Understand where masonry control joints must be placed before any work begins.